How to Determine Peach Ripeness

Hey, I’m Desmond Layne, Peach Specialist at Clemson University.  Welcome to the Clemson Tiger Peach Network.

Welcome back to Everything About Peaches!  Our second episode is “How To Determine Peach Ripeness.”  Few things are more satisfying than biting into a delicious, juicy, tree-ripened peach.  However, as a grower, how can you be sure to pick fruit at the optimal stage of ripeness for your market?  Today, we will give you some information to ensure that you meet that goal.

As peaches ripen on the tree, several changes occur that can be visibly seen or easily measured using some simple tools.  This can help to ensure that you “get it right” at harvest time.

One change is in skin coloration.  Unripe fruits have green skin.  Skin coloration has two different aspects.  One is called background color and the other is called blush.  Background color will typically change from green to yellow or orange as fruits ripen.  Blush is the red coloration that occurs in response to sunlight.  Many traditional “southern” peaches will be yellow with a red blush when ready to be picked.  However, some newer, solid-red cultivars will be completely red before they are at optimal maturity – so, for them, other measures are necessary to ensure accurate harvest timing.  

A second change is in size or diameter.  During the last two weeks prior to harvest, peaches go through a “final swell” stage where individual cells in the fruit expand as they are filled with water, sugars, and acids.  Diameter can be easily measured using a hand caliper in inches.  Diameter can be affected by the crop load on the tree, how recently the tree received rainfall and irrigation, or the genetics of the cultivar itself.  Some cultivars are genetically larger than others.  Market price is generally determined by size where larger fruits are favored.

A third change is in fruit firmness.  Enzymatic activity in the flesh causes the tissue to soften, especially for melting flesh cultivars.  This can be easily measured using a penetrometer.  Simply remove a small amount of skin from either side of the fruit and press the penetrometer into the flesh.  The amount of force (pounds per square inch) needed to penetrate the fruit indicates how firm it is.   You can read on the yellow scale to see how many pounds of pressure the fruit has.  This fruit is eight pounds.  Tree-ripened fruit for the roadside market will typically be softer, usually between 2-4 pounds of pressure. As you can see, this one has 2 pounds. Fruit to be shipped long-distance may be as firm as 10 pounds of pressure when harvested so that the fruit are not easily bruised in packing or transit.  Mature fruits stored at room temperature will generally decrease by about 2 pounds of pressure each day.

A fourth change is in soluble solids concentration also known as brix.  Now, I am not talking about bricks used in building a house. ☺  Rather, brix refers to the sugar concentration of the extracted juice.  Brix can be easily measured using an instrument called a refractometer.  By extracting a small sample of juice and placing a drop on the glass surface of this instrument, you can read a line to determine what the concentration really is.  For peaches, this number can be between 8 to more than 20.  A higher number is better and numbers exceeding 12 are considered optimal for high consumer acceptance.

Finally, two other changes, a little more difficult to quantify, are aroma and taste.  Many cultivars will have a strong “peachy” aroma as they ripen.  For more traditional yellow-fleshed peaches there will be a characteristic strong tangy (acid) flavor that is complimented with good sweetness.  However for newer, sub-acid and particularly white-fleshed cultivars, there will be sweetness like honey with little or no acidity or tang at optimal maturity. These fruits taste characteristically bland.  Asian peoples particularly like white-fleshed cultivars because they are common in their homelands.

For the large-scale, commercial shipper, scientists and engineers have developed highly sophisticated computerized equipment that can be used to automatically and accurately measure fruit size, color and even estimated sugar content on the rapidly moving lines in a packing house.  However, if you don’t have access to that kind of equipment, you can use these simple tools that we showed you today to measure the same exact thing.   

To review, optimal ripeness at harvest depends on whether you are providing tree-ripened fruit to the local market or less mature fruit to a distant market. Remember, all peach cultivars are not to be treated the same – for example, those with solid red skin color before they are mature may need multiple measures to confirm optimal maturity.  Changes in peach maturity are easily measured with some simple tools that you can buy and have on hand to use yourself.  You can actually see changes in background color and blush with your own eyes. Changes in size can be measured easily using a hand-held caliper. Firmness can be easily measured using a penetrometer, and brix, or sugar concentration, can be easily measured using a refractometer.  Generally, aroma and taste will improve significantly in the last two weeks before harvest.  Finally, because all of the peaches on a given tree don’t ripen at the same time, each time you harvest, pick only those fruits that are at optimal maturity to ensure the satisfaction of your customers.  Mmmm.  Just right!

For more information on peaches, you can view my Clemson peach website at www.clemson.edu/hort/peach. And to read my regular peach columns for the American Fruit Grower magazine, visit their website at www.growingproduce.com.